Dimethylbiphenyl (DMBP) and other dialkylbiphenyls are useful intermediates in the production of a variety of commercially valuable products, including polyesters and plasticizers for PVC and other polymer compositions. For example, DMBP can readily be converted to an ester plasticizer by a process comprising oxidation of the DMBP to produce the corresponding mono- or dicarboxylic acid followed by esterification with a long chain alcohol. However, for certain uses, it is important to reduce the level of 2,X′ DMBP (where X′ is 2′, 3′ and 4′) isomers in the product since, for example, diphenate esters having substitution on the 2-carbons tend to be too volatile for use as plasticizers.
As disclosed in our co-pending U.S. Ser. Nos. 14/201,287 and 14/201,224, both filed Mar. 7, 2014, dimethylbiphenyl may be produced by hydroalkylation of toluene followed by dehydrogenation of the resulting (methylcyclohexyl)toluene (MCHT). However, even using a selective molecular sieve catalyst for the hydroalkylation step, this process tends to yield a mixture of all six DMBP isomers, namely 2,2′, 2,3′ 2,4′, 3,3′, 3,4′ and 4,4′ DMBP, in which the 2,X′ DMBP isomer content may be 20% by weight or more of the total DMBP product. Moreover, 2,2′, 2,3′ and 2,4′ DMBP cannot be completely separated from unreacted MCHT by distillation due to an overlap in their vapor-liquid equilibrium properties. There is, therefore, interest in developing a process for converting 2,X′ DMBP isomers into 3,3′, 3,4′ and 4,4′ DMBP and especially a process which effects such a conversion with minimal loss of any unconverted MCHT which may be present in the DMBP feed.
The production of biphenylesters is based on the initial steps of aromatic hydroalkylation (see U.S. Ser. No. 14/201,226, U.S. Ser. No. 14/201,287, and U.S. Ser. No. 14/201,224, filed Mar. 7, 2013), followed by dehydrogenation to produce biphenyls or alkylbiphenyls (see U.S. Ser. No. 14/164,889, filed Jan. 27, 2013, U.S. Ser. No. 14/201,287, and U.S. Ser. No. 14/201,284, filed Mar. 7, 2013).